Any credible source of bioenergy should not only be economically viable but also environmentally sustain­able. The economic and environmental impacts of any source of bioenergy, including biolipids from microal­gae, will usually be measured in terms of energy return on energy investment (EROI) and/or GHG emissions. These economic and environmental impacts of biofuels and microalgae biofuels in general have been hotly debated in recent years. A number of life cycle analyses (LCAs) have been undertaken with seemingly conflict­ing results (Benemann et al., 2012; Liu et al., 2011; Resurreccion et al., 2012; Sun et al., 2011). Similar disparities arose in the case of second-generation bio­fuels such as corn ethanol before the introduction of the Energy and Resources Group (ERG) Bioenergy Meta-Model (Farrell et al., 2006). The results of reported LCA analyses are hindered by the lack of fully inte­grated commercial-scale microalgae to bioenergy sys­tems from which to obtain accurate measurements. Estimates are based on projections from laboratory — and pilot-scale tests, as well as some commercial data. Despite these facts an overall meta-analysis concluded that algae-based biodiesel would result in energy con­sumption and GHG emissions on par with terrestrial al­ternatives (Liu et al., 2011). In this study the authors consider a microalga-based bioenergy system whereby CO2 and nitrogen for microalgae cultivation are recycled from waste streams and the microalgae coproducts are used for further bioenergy production in the form of methane. This concept of an integrated "biorefinery" has been proposed previously (Borowitzka, 1995, 1999; Chisti, 2007; Martin and Grossmann, 2012). As alluded above, the "biorefinery" concept envisages the main in­puts into the cultivation process such as carbon, nitro­gen and phosphorus being supplied through various waste streams. Similarly, the microalgae product result­ing from cultivation could be fully "refined" into a num­ber of outputs including biolipids for bioenergy, biolipids for nutraceutical applications, proteins for an­imal feeds, sugars for bioethanol production, etc. At pre­sent, where fully commercial scale cultivation of microalgae and conversion to fuel alone is still not economically feasible, the "biorefinery" concept appears to offer the best short to medium term path to scale-up.

In addition to the potential economic and environ­mental advantages of using microalgae-derived bio­lipids, the properties of the resulting biodiesel product are also worth considering. As detailed later in this chapter, biodiesel is produced by transesterification of the biolipids from an appropriate feedstock. Much like the plant — and animal-based biolipids discussed previ­ously, the profile of the microalgae-derived biolipids that undergo transesterification will ultimately deter­mine the quality of the biodiesel product. This profile will include the level of polyunsaturated fatty acids (PUFAs), the level of FFAs and the level of TAGs. Although the lipid profile of microalgae varies among species and even among the same species under different conditions of growth, approximately 80% of the lipid content of microalgae, in general, will be made up of storage lipids in the form of TAGs. TAGs are made up of three fatty acid chains, usually with a chain length of C14 to C22 for microalgae-derived bio­lipids, joined to glycerol through three ester bonds (Scott et al., 2010). These TAGs can be easily transesterified in the presence of methanol, as described later in the chap­ter, to fatty acid methyl esters (FAMEs), which make up biodiesel. The presence of FFAs, however, results in the formation of soaps during transesterification in the pres­ence of a base catalyst such as NaOH. This increases the downstream processing required to produce a finished biodiesel product. Similarly, the presence of PUFAs in biolipids derived from some microalgae species can cause tar formation resulting from fatty acid chains cross-linking (Burton et al., 2009). A high PUFA content could also mean that a biodiesel product would not pass European standards for biodiesel (EN14214), which de­mand the content of FAMEs with four or more double bonds to be below 1% mol (Knothe et al., 2005). Other properties that have been considered with regard to other feedstocks mentioned in this chapter include the cloud point, the cetane number and the oxidation stabil­ity of the biodiesel fuel. It has been suggested that bio­diesel from microalgae oils may face significant performance problems regarding cold flow and oxida­tive stability in particular (Knothe, 2011); however, exceptions to this observation may apply to some micro­algae such as Trichosporon capitatum. Also, in a recent study, biodiesel derived from the microalgae Chaetoceros gracilis was found to generate similar torque and power to soy-derived biodiesel. In terms of emissions, the C. gracilis-derived biodiesel also produced less CO, NOx and hydrocarbons than petroleum diesel (Wahlen et al., 2012).

It is clear that the potential for algae to supply a sus­tainable source of biolipid for transportation fuel and other forms of bioenergy is not in doubt. However, there remain technical, economic and environmental chal­lenges to be overcome. In a recent report by the National

Research Council in the United States entitled, "Sustain­able development of algal biofuels" a number of sustain­ability concerns were highlighted. These included EROI; GHG emissions and resource usage such as land, water, nitrogen, phosphorus, and carbon dioxide (National Research Council, 2012). None of these concerns, however, were considered a "definitive barrier to sus­tainable development of algal biofuels". This is because a number of strategies have already been implemented to tackle these challenges. As mentioned previously the use of wastewater streams can drastically reduce resource usage and GHG emissions as well as greatly in­crease EROI. Current projects, at industrial scale, such as Sapphire Energy’s "Green Crude Farm" (Sapphire Energy, 2013) aim to have a capacity of 1 million gallons per year of finished biofuel product. It is predicted that this will result in a 60—70% reduction in GHG emissions compared to traditional fossil crude oil, which, if achieved, will make the potential of microalgae — derived biofuel a very definite reality.